The invention disclosed herein is for improving the resolution or precision of analog-to-digital converters. For example, the invention makes it possible to obtain with a nominally 8-bit converter substantially the precision that could otherwise only be obtained with a 12-bit or greater converter.
As is well known, an analog-to-digital converter (ADC) takes successive samples of an analog signal and converts them to digital values. It is also known that the cost of an ADC increases in proportion to its conversion speed and in proportion to the number of bits into which it quantizes each sample. When high resolution is required, the electronic circuit designer is compelled to select a more precise and costly converter.
The accuracy and precision of an integrated circuit ADC, like most if not all of measuring instruments, depends on the number of parts into which the measurement unit is subdivided on the measuring scale. Consider an ordinary straight scale one meter in length on which the smallest subdivisions are marked one centimeter apart, that is, the centimeters are not divided into millimeters. If this scale is laid with its zero end at the left edge of an object and the opposite end of the object falls between centimeters 30 and 31, for example, the true measurement can only be guessed so precision within one centimeter is the best obtainable. Comparable conditions apply to ADCs. If a ADC has the capability for resolving an analog signal sample into an 8-bit binary number or 256 levels in decimal number terms, one part in 256 is the best resolution obtainable when a ADC is used in the conventional way ADCs always have some error as is known. For instance, if an analog signal sample of a fixed amplitude were digitized repeatedly by a given ADC, the resulting binary numbers would most likely differ in their least significant bits and probably also in higher order bits.
In some mechanical measuring devices, the resolution of the finest division on the scale that can be read with certainty is improved by use of a vernier. The verneir scale consists of a number of divisions corresponding to the fraction of the main division required. Suppose a one meter scale is graduated in millimeters and one desires to measure to within tenths of a millimeter. The length of the vernier scale is made one less than a similar number of divisions on the main scale so on the vernier scale ten divisions is equal to nine divisions of the main scale. Thus, each vernier division may be one tenth short of a main scale division. Conversely, each main scale division is ten-ninths of a vernier scale division. The vernier scale is mounted for sliding next to the main scale. Thus, to read to tenths of the main scale, one progresses along the vernier scale from zero until a graduation mark on the vernier scale aligns with a graduation mark of the main scale. For example, if the third vernier graduation above zero aligned with one on the main scale, an amount equal to three-tenths of a main scale division would be added to the nearest accurately readable integer number marked under the fraction or seven-tenths of a division would be subtracted from the next integer number over the fractional part. What one is actually doing is adding or subtracting a constant offset, one-tenth in this example, as one progresses along the vernier scale, summing the number of vernier scale divisions and dividing by the number (10 in this example) of parts into which a main scale division is to be divided.